Field of the Invention
The present invention relates to age and heat stabilized photovoltaic solar cells. More particularly, the invention relates to heterojunction photovoltaic cells having a stabilizing metal electrode.
The preparation of thin film photovoltaic cells is well known. However, a great deal of research effort is still expended in the development and improvement of photovoltaic cells in order to provide greater cell efficiency and economic feasibility. In particular, there is a continuing need for improvement of the age and heat stability of heterojunction photovoltaic cells such as cadmium sulfide photovoltaic cells.
Solar cells prepared from CdS as well as other solar cells prepared from compound semiconductors are known to degrade during operation. The degradation in cell efficiency is thought to occur by thermally activated diffusion. The Cu.sub.x S-CdS solar cell is particularly sensitive to degradation through thermal effects. Cu.sub.x S indicates a copper sulfide composition wherein the value of x is in the range of 1.95 to 2.0. The thermal and age degradation of this cell is thought to be caused by diffusion of copper from the Cu.sub.x S layer into the CdS layer, and to changes in the composition of the Cu.sub.x S layer.
When the Cu.sub.x S-CdS solar cell is fabricated from thin films, without any subsequent heat treatment, the cell has a low efficiency owing to a low open circuit voltage and a poor I-V curve. A heat treatment of such cells has been found to improve the efficiency, and this heat treatment is now conventionally used in the preparation of such cells. For example, T. S. te Velde and J. Dieleman (Phillips Res. Reports 28,573 (1973)), using single crystal CdS, found that oxygen and hydrogen heat treatments influence the efficiency of the Cu.sub.x S-CdS cell.
A. Rothwarf (2nd E. C. Photovoltaic Solar Energy Conference, R. Van Overstraelen and W. Palz, Eds., Reidel Publ. Co., 1979, page 370) has reported that hydrogen and air (oxygen) heat treatments modify the conductivity of the Cu.sub.x S layer, and that the optimum conductivity corresponds to Cu.sub.x S with x in the range from 1.9995 to 1.99995.
W. J. Biter (Solar Energy, J. B. Berkowitz and I. A. Lesk, Eds., Electrochem. Soc., Princeton, N.J. P. 337 (1976)) has shown that thin films of metal evaporated onto the Cu.sub.x S layer after heat treatment cause a change in the efficiency of the cell. For example, a thin layer of copper improved the cell efficiency, whereas thin layers of Au, Ag, Zn, Cd, Al, In, Ni and Sn caused a decrease in the cell efficiency. These changes were thought to be caused by diffusion of the evaporated metal through the Cu.sub.x S layer, causing a change at the heterojunction interface.